Abstract:

The invention refers to salts of at least one imidazole of the general
formula (I), in which R1, R2, R3 and R4 are the same
or different and denote hydrogen, an alkyl residue having 1 to 20,
preferably 1 to 10, more preferably 1 to 4 carbon atoms, or a substituted
or unsubstituted aryl or arylalykl residue having 6 to 10 carbon atoms,
and at least one aliphatic or aromatic mono- or dicarboxylic acid. The
molar ratio of carboxylic acid to imidazole, based on the functionality
of the acid, is 1:1.1 to 1:6. The invention also relates to a method for
manufacturing the imidazole salts, to their use, and to epoxy resin
compositions containing said salts.

Claims:

1. A salt of at least one imidazole of the general formula ##STR00003## in
which R1, R2, R3 and R4 are the same or different and
denote hydrogen, an alkyl residue having 1 to 20 carbon atoms, or a
substituted or unsubstituted aryl or arylalkyl residue having 6 to 10
carbon atoms, and at least one aliphatic or aromatic mono- or
dicarboxylic acid, wherein the molar ratio of carboxylic acid to
imidazole, based on the functionality of the carboxylic acid, is 1:1.1 to
1:6.

2. A salt according to claim 1, wherein the imidazole is an unsubstituted
imidazole, an alkyl-substituted imidazole having one or more substituents
having 1 to 6 carbon atoms, or an aryl-substituted imidazole having one
or more substituents having 6 to 8 carbon atoms.

3. A salt according to claim 1, wherein the imidazole is selected from the
group consisting of imidazole, N-methylimidazole and
1,2-dimethylimidazole.

4. A salt according to claim 1, wherein the carboxylic acid is selected
from the group consisting of aliphatic and aromatic mono- and
dicarboxylic acids having 1 to 20 carbon atoms.

5. A salt according to claim 1, wherein the carboxylic acid is selected
from the group consisting of unsaturated substituted and unsubstituted
monocarboxylic acids having 3 to 5 carbon atoms, unsaturated substituted
and unsubstituted dicarboxylic acids having 4 to 8 carbon atoms,
saturated substituted and unsubstituted monocarboxylic acids having 1 to
5 carbon atoms and saturated substituted and unsubstituted dicarboxylic
acids having 2 to 5 carbon atoms,

6. A salt according to claim 1, wherein the carboxylic acid is selected
from the group consisting of 2-ethylhexanoic acid, salicylic acid,
dodecanoic acid, benzoic acid, and succinic acid.

7. A salt according to claim 1, wherein the molar ratio of carboxylic acid
to imidazole, based on the functionality of the carboxylic acid, is 1:2
to 1:4.

8. A method for manufacturing a salt according to claim 1, comprising
reacting at least one imidazole of the general formula ##STR00004## in
which R1, R2, R3 and R4 are the same or different and
denote hydrogen, an alkyl residue having 1 to 20 carbon atoms, or a
substituted or unsubstituted aryl or arylalykl residue having 6 to 10
carbon atoms, with at least one aliphatic or aromatic mono- or
dicarboxylic acid at a molar ratio of carboxylic acid to imidazole of
1:1.1 to 1:6, based on the functionality of the carboxylic acid, at a
temperature between 20.degree. C. and 120.degree. C.

9. The method according to claim 8, wherein the molar ratio of carboxylic
acid to imidazole, based on the functionality of the carboxylic acid, is
1:2 to 1:4.

10. A method of curing a polyepoxide, comprising combining said
polyepoxide with at least one salt in accordance with claim 1 to form a
composition and heating said composition.

11. An epoxy resin composition comprising at least one polyepoxide having
at least two epoxy groups per molecule and at least one salt of at least
one imidazole of the general formula ##STR00005## in which R1,
R2, R3 and R4 are the same or different and denote
hydrogen, an alkyl residue having 1 to 20 carbon atoms, or a substituted
or unsubstituted aryl or arylalkyl residue having 6 to 10 carbon atoms,
and at least one aliphatic or aromatic mono- or dicarboxylic acid, the
molar ratio of carboxylic acid to imidazole, based on the functionality
of the carboxylic acid, being 1:1.1 to 1:6.

12. An epoxy resin composition according to claim 11, wherein the
proportion of the salt is 1 to 10 wt. %, based on the total weight of
epoxy resin composition.

13. The epoxy resin composition according to claim 11, wherein the
proportion of the salt is 0.01 to 40 wt.%, based on the total weight of
epoxy resin composition.

Description:

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001]This application is a continuation under 35 USC Sections 365(c) and
120 of International Application No. PCT/EP2006/004160, filed 4 May 2006
and published 7 Dec. 2006 as WO 2006/128542, which claims priority from
German Application No. 102005024255.3, filed 27 May 2005, each of which
is incorporated herein by reference in its entirety.

FIELD OF THE INVENTION

[0002]The present invention relates to salts of at least one imidazole of
the general formula

##STR00001##

in which R1, R2, R3 and R4 are the same or different
and denote hydrogen, an alkyl residue having 1 to 20, preferably 1 to 10,
more preferably 1 to 4 carbon atoms, or a substituted or unsubstituted
aryl or arylalykl residue having 6 to 10 carbon atoms, and at least one
aliphatic or aromatic mono- or dicarboxylic acid. The invention further
relates to a method for manufacturing said salts, to their use as
catalysts in the curing of polyepoxides, and to epoxy resins based on
polyepoxides that contain said salts.

DISCUSSION OF THE RELATED ART

[0003]Epoxy resins are aliphatic, cycloaliphatic, or aromatic oligomers
that contain oxirane groups and can be crosslinked with resins to yield
thermoset plastics. Most epoxy resins are glycidyl ethers of bisphenol A
derived from the reaction of bisphenol A with epichlorohydrin. There are
also epoxy resins based on epoxidized phenol-formaldehyde or
cresol-formaldehyde resins, hydantoin, hexahydrophthalic acid, and the
like. The resins can be cured cold with polyfunctional amines, or at high
temperature using multifunctional carboxylic acids or carboxylic acid
anhydrides. Ester and ether structures are formed as this
high-temperature curing proceeds. The possibility also exists of curing
epoxy resins by anionic polymerization. Epoxy resins are used for a very
wide variety of purposes, for example as adhesives, coatings, for
components, large containers, etc. When they are used as engineering
structural materials, they are usually reinforced with glass fibers or
carbon fibers.

[0004]A number of polyepoxides that contain at least two 1,2-epoxy groups
per molecule are suitable s epoxies. The epoxy equivalent of these
polyepoxides can vary from 150 to 4000. The polyepoxides can in principle
be saturated, unsaturated, cyclic or acyclic, aliphatic, alicyclic,
aromatic, or heterocyclic polyepoxide compounds. Examples of suitable
polyepoxides include the polyglycidyl ethers, which are manufactured by
reacting epichlorohydrin or epibromohydrin with a polyphenol in the
presence of alkali. Polyphenols suitable for this are, for example,
resorcinol, catechol, hydroquinone, bisphenol A
(bis-(4-hydroxyphenyl)-2,2-propane), bisphenol F
(bis-(4-hydroxyphenyl)methane), (bis-(4-hydroxyphenyl)-1,1-isobutane),
4,4'-dihydroxybenzophenone, bis(4-hydroxyphenyl)-1,1-ethane, and
1,5-hydroxynaphthalene.

[0005]Further polyepoxides that are suitable in principle are the
polyglycidyl ethers of polyalcohols or diamines. These polyglycidyl
ethers are derived from polyalcohols such as ethylene glycol, diethylene
glycol, triethylene glycol, 1,2-propylene glycol, 1,4-butylene glycol,
triethylene glycol, 1,5-pentanediol, 1,6-hexanediol, or
trimethylolpropane.

[0007]Further epoxies are derived from the epoxidation products of
olefinically unsaturated cycloaliphatic compounds or of natural oils and
fats.

[0008]The epoxy resins derived from the reaction of bisphenol A or
bisphenol F and epichlorohydrin (DGEBA or DGEBF) are very particularly
preferred. Mixtures of liquid and solid epoxy resins are usually used,
the liquid epoxy resins by preference being based on bisphenol A and
having a sufficiently low molecular weight.

[0009]In the manufacture of composite materials, fast cycle times for
efficient series production of the fiber-reinforced epoxy-resin
components are necessary. These have hitherto been achieved, however,
only by means of the prepreg technique, at relatively high temperatures
of 140° to 160° C. The use of so-called wet resin
techniques eliminates one production step, namely prepregging.
Homogeneous resin systems are required, however, and rapid curing is
difficult. To achieve good wetting of the fibers or fabric, the viscosity
of the resins is lowered by elevating the temperature to 60° C. to
80° C. Crosslinking should not yet be occurring at this
temperature. On the other hand, it is desirable to achieve rapid curing
by way of a slight temperature increase to 80° C. to 120°
C. Even when adhesively bonding metal parts using epoxy resins, it is
desirable on the one hand to decrease the viscosity of the resins by
temperature elevation, and on the other hand to achieve rapid curing by
means of a slight temperature rise.

[0010]It is known that the crosslinking of epoxy resins can be accelerated
by anionic polymerization at elevated temperature using imidazole salts.
These imidazole salts are referred to in the literature in some cases as
hardeners and in some cases as catalysts.

[0014]Lastly, U.S. Pat. No. 3,356,645 also describes imidazole salts of
organic acids as hardeners or catalysts for curing epoxy resins.
Monocarboxylic acids having 1 to 8 carbon atoms, and lactic acid, are
recited as organic acids.

[0015]In the aforesaid patent documents, imidazole, which can optionally
be substituted, is mixed with the acid at a 1:1 molar ratio, or the acid
is used at an excess with respect to the imidazole, for manufacture of
the imidazole salts.

[0016]The known hardeners or catalysts allow the epoxy resins mixed with
them to be stored at low temperature, and upon elevation of the
temperature, curing of the resins occurs by crosslinking. These known
systems are, however, unsatisfactory in one respect: the temperature
difference between the temperature at which the resins equipped with the
catalyst can be stored without crosslinking, and the temperature at which
effective crosslinking occurs, is relatively large.

[0017]It is an object of the present invention to reduce this temperature
difference and to describe catalysts or hardeners for epoxy resins such
that on the one hand no curing occurs at up to approximately 80°
C., and on the other hand effective and rapid curing takes place with
only a slight temperature elevation to approximately 100° C.

BRIEF SUMMARY OF THE INVENTION

[0018]It has been found, surprisingly, that this object can be achieved by
imidazole salts of organic acids that are manufactured with an excess of
imidazole. The subject of the present invention is therefore salts of the
kind cited initially that are characterized in that the molar ratio of
carboxylic acid to imidazole, based on the functionality of the acid, is
1:1.1 to 1:6, by preference 1:2 to 1:4. These base (imidazole)-rich salts
act as latent accelerators for epoxy resins, and they are very well
suited for rapid processing of epoxy resins. The salts are liquid at room
temperature and can easily be mixed with epoxy resins. This mixture can
be produced before use, and can heated without difficulty to temperatures
of up to approximately 80° C., for example in order to achieve
complete wetting of the fibers when manufacturing fiber-reinforced shaped
parts. Surprisingly, the residual organic acid ions also positively
influence the material properties of the materials, such as glass
transition temperature, water absorption, and elasticity. A slight
increase in temperature to approximately 100° C. causes a rapid
crosslinking to occur, i.e., curing is accelerated by a factor of >2
as compared with the 1:1 salts.

[0021]Preferred imidazoles are unsubstituted imidazole, alkyl-substituted
imidazoles having substituents that have 1 to 6 carbon atoms, and
aryl-substituted imidazoles having substituents that have 6 to 8 carbon
atoms.

[0022]The carboxylic acids can be selected from the group made up of
substituted or unsubstituted, saturated or unsaturated monocarboxylic
acids having 3 to 22 carbon atoms, substituted or unsubstituted,
saturated dicarboxylic acids having 2 to 36 carbon atoms, substituted or
unsubstituted, unsaturated dicarboxylic acids having 4 to 36 carbon
atoms, and substituted or unsubstituted aromatic mono- or dicarboxylic
acids.

[0024]The invention also relates to a method for manufacturing the
imidazole salts according to the present invention, which method is
characterized in that at least one imidazole of the general formula

##STR00002##

in which R1, R2, R3 and R4 are the same or different
and denote hydrogen, an alkyl residue having 1 to 20, preferably 1 to 10,
more preferably 1 to 4 carbon atoms, or a substituted or unsubstituted
aryl or arylalkyl residue having 6 to 12 carbon atoms, and at least one
aliphatic or aromatic mono- or dicarboxylic acid, are reacted with one
another at a molar ratio of carboxylic acid to imidazole of 1:1.1 to 1:6,
by preference 1:2 to 1:4, based on the functionality of the acid, at a
temperature between 20° C. and 120° C.

[0025]The invention also relates to the use of the imidazole salts as
catalysts in the curing of polyepoxides and to epoxy resins based on
polyepoxides, having at least two epoxy groups per molecule, that contain
the imidazole salts according to the present invention. The proportion of
the imidazole salts is advantageously 0.01 to 40 wt %, preferably 1 to 10
wt %, based on the total weight of epoxy resin and salt.

[0026]The invention is explained in further detail below with reference to
exemplifying embodiments.

Manufacture of the Imidazole Salts

[0027]The imidazole salts were manufactured by reacting the starting
materials indicated in the following table, at the molar ratio indicated.
For this, the imidazole components were finely powdered and mixed with
the acid component with vigorous agitation. Agitation was continued at
room temperature for 6 to 12 hours until a homogeneous phase was
obtained.

[0028]Upon elevation of the temperature to 100° C., the reaction
proceeded within 30 to 60 minutes.

[0029]The products were obtained as clear, pale-yellow to golden-yellow
liquids that in some cases had an oily character.

[0030]For comparison, in two experiments 2-ethylhexanoic acid and
1,2-dimethylimidazole and imidazole, respectively were reacted at a 1:1
molar ratio.

Use of the Imidazole Salts

[0031]The imidazole salts were introduced, at a proportion of 5 wt % based
on the total weight of the mixture, into an epoxy resin formulation.

[0033]Using an adhesive manufactured in this fashion, cleaned and
degreased ZE steel panels of dimensions 100×25 mm (adhesive bonding
area 25×10 mm) were adhesively bonded, and cured for 10 minutes at
80° C. and 100° C. The adhesively bonded panels were then
investigated in terms of the tensile shear strength of the adhesive bond
(ascertained per DIN 53283, "Determination of the adhesive strength of
single-lap jointed adhesive bonds" at a rate of 100 mm/min).

[0034]At 80° C., the tensile shear strength value for all specimens
was 0 MPa, i.e., no curing occurred at this temperature.

[0035]At 100° C., the tensile shear strength of the specimens
according to the present invention was 1.4 to 8.3 MPa. The comparison
specimens displayed tensile shear strength values of 0.1 and 0.4 MPa. The
results show that the base-rich imidazole salts according to the present
invention cause a considerable acceleration in curing to occur even at
100° C.